In FM communication circuits, there occurs an intermodulation noise due to a linear distortion, such as an amplitude or phase distortion in an electric wave propagation path, a waveguide system or an FM signal amplifying system, resulting in the degraded signal quality; it is known in the art that the most forceful factor of such degradation is the phase distortion. The phase distortion is caused by the presence of an echo in the transmission system, and in the waveguide system and in the amplification system, generation of the echo is steady, so that phase equalization can be achieved by the use of a fixed phase equalizer. However, in a multipath-propagation path, especially in an over the horizon scattering propagation path, an oversea propagation path, a mountainous diffraction propagation path or the like, the phase distortion is often accompanied by fading, so that diversity reception is usually employed to improve the phase distortion; this is very uneconomical.
The "Automatic Phase Equalization System in FM communication circuits" (Japan. Pat. Appln. Ser. No. 57130/78) previously proposed by the present inventors is intended to maintain the overall phase characteristic substantially linearly at all times by detecting a phase distortion from a product component of various pilot signals included in a demodulated base-band signal and automatically controlling a phase equalizer by the detected output. In the invention of this prior application, in the detection of the phase distortion, coefficients of respective degrees in a variable phase equalizer of a power series characteristic provided in the former stage of demodulation of the FM signal, that is, in the intermediate-frequency stage, are slightly changed in amplitude at a low frequency to thereby vary the envelope of the product component, and from the phase of the variation, the polarity of the coefficient of the power series is detected. Accordingly, a control signal therefor cannot be obtained unless the envelope signal, that is, a low-frequency signal is phase detected and smoothed, so that the response speed as an automatic control system depends on the frequency of this low-frequency signal; therefore, it is desirable to select this frequency as high as possible. However, the rise in frequency is limited by the band width of a band-pass filter used for extracting the product component; namely, the response speed of the automatic control system is held low in correspondence to the abovesaid smoothing, resulting in incapability of following up the rapid selective fading.